1. Technical Field
The present invention relates to a glossing device, a fixing device, and an image forming apparatus incorporating the same, and more particularly, to a fixing device that processes a toner image with heat and pressure on a recording medium for imparting gloss, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of these features, which incorporates such a fixing device with a glossing capability.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process may be followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and setting the toner with heat and pressure.
Modern image forming apparatuses accommodate a wide range of printing applications with different levels of image quality, using various types of recording media. In particular, printing of photographs and computer-generated images, which typically contain a wide, complete range of visible colors, necessitates a higher level of image quality and uniformity of image gloss than is required for conventional monochrome image formation.
Various techniques have been proposed to meet a growing demand for printers with high-gloss, high-quality imaging performance. Some such techniques employ a special, transparent toner, called “clear toner”, for creating a transparent glossy effect on those areas of a recording medium where no color toner is deposited; others address duplex printing with a uniform, glossy finish on both sides of a recording medium. Among these, several techniques are directed to development of a more sophisticated fixing process.
Structurally, a fixing device with a glossing capability may be constructed of an endless rotary belt on which a recording medium is conveyed while subjected to heat and pressure. The endless belt is looped for rotation around multiple parallel rollers, including a heated roller and a separator roller, with a pressure roller disposed opposite the heated roller via the belt to form a fixing nip therebetween. During operation, a recording medium is conveyed through the fixing nip to process a toner image under heat and pressure. After passage through the fixing nip, the recording medium closely contacts the belt as the belt moves from the heated roller toward the separator roller, and separates from the belt as the belt passes around the separator roller.
For example, a belt-based fixing system has been proposed which includes a thermal pre-fixing unit and a gloss adjustment unit. The pre-fixing unit consists of a pair of opposed heated rollers pressing against each other to form a pre-fixing nip therebetween. The gloss adjustment unit consists of a smooth, endless rotary belt entrained around a pair of motor-driven and idler rollers, with a pressure roller opposite the motor-driven roller to form a main fixing nip therebetween.
In this fixing system, a recording medium is initially passed through the pre-fixing unit, which renders an unfixed powder toner image into a semi-fluid, soft pliable state. After pre-fixing, the recording medium is conveyed to the gloss adjustment unit with the toner image pressed against the endless belt, which imparts gloss to the toner image as the molten toner gradually cools and solidifies while conforming to the smooth surface of the belt. The gloss adjustment unit adjusts glossiness of the toner image by adjusting a distance or duration during which the toner image travels on the belt downstream from the fixing nip.
To date, belt-based fixing devices are designed with a belt cooler for cooling an endless rotary belt during conveyance of a recording medium downstream from a fixing nip, so as to provide uniform cooling and proper separation of the recording medium from the belt after fixing and glossing a toner image thereon.
For example, one known image heating device employs a non-contact belt cooler, such as a cooling fan, disposed apart from an endless rotary belt for cooling the belt by air flow or forced convection.
In this image heating device, the endless rotary belt is entrained around fixing and separator rollers as well as a heater interposed between the rollers, with a backup, pressure roller disposed opposite the heater via the belt to form a nip therebetween. The belt assembly also includes a pair of electrostatically charged rollers disposed opposite each other via the belt downstream from the nip to impart electrostatic charge to the belt and a recording medium passing therethrough, which prevents premature separation of the recording medium from the belt. The cooling fan is disposed downstream from the heater to cool the belt and the recording medium after passing through the nip.
Another fixing device is known employing a heat sink, instead of a non-contact belt cooler, disposed in contact with an endless rotary belt for cooling the belt through direct contact with the heat absorbing surface.
In this fixing device, the endless rotary belt is entrained around multiple rollers, including a heated fixing roller and a separator roller, with a pressure roller disposed opposite the fixing roller via the belt to form a fixing nip therebetween. The belt assembly also includes an additional, heated pre-fixing roller disposed inside the loop of the belt upstream from the fixing nip, with a backup roller disposed opposite the pre-fixing roller to form a pre-fixing nip therebetween. The heat sink is provided inside the loop of the belt between the fixing roller and the separator roller, with an outer surface of the heat sink parallel to, and in contact with, the belt surface to cool the belt after heating through the pre-fixing and fixing nips.
Still another fixing device is known employing a cooling fan in combination with a cooler backup roller pressing an endless rotary belt against the fan to increase efficiency in cooling the belt.
In this fixing device, the endless rotary belt is entrained around multiple rollers, including a heated fixing roller and a separator roller, with a pressure roller disposed opposite the fixing roller via the belt to form a fixing nip therebetween. The cooling fan is provided inside the loop of the fixing belt between the fixing roller and the separator roller, with a cooler backup roller disposed opposite the cooling fan via the fixing belt to press the belt against the cooling fan from outside the loop of the fixing belt.
Yet still another fixing device is known employing a pair of cooling devices disposed opposite each other to increase cooling efficiency while allowing for efficient media conveyance through the fixing process.
This fixing device includes a pair of opposed belt assemblies, each of which includes an endless rotary belt entrained around a heated roller and a separator roller, with the two heated rollers of the opposed sides pressing against each other via the belts to form a first fixing nip therebetween, and the two separator rollers of the opposed sides pressing against each other via the belts to form a second fixing nip therebetween. The pair of cooling devices is provided, one for each of the belt assemblies, each cooling device located inside the loop of the fixing belt between the heated roller and the separator roller.
Although generally successful for their intended purposes, the approaches depicted above have several drawbacks.
For example, the image heating device using a cooling fan has a limitation in that it cannot accommodate high processing speeds as required in today's high-speed printers, insofar as a non-contact cooler employing air flow or convection cannot remove heat as efficiently and immediately as a contact cooler.
Also, the fixing device using a heat sink is not satisfactory in terms of cooling efficiency because deploying the heat sink and the belt parallel to each other translates into a relatively loose contact between the heat sink and the belt, resulting in insufficient heat transfer from the belt to the heat sink.
Further, the fixing device using a cooling fan with a cooler backup roller is not practical where pressing the belt against the cooling fan adversely affects durability of the belt assembly. That is, striking against an edge of the cooling device can strain the belt in its longitudinal, conveyance direction, while sliding against the cooling device can cause increased wear and tear on the belt surface. Moreover, use of the cooler backup roller entails a risk of damaging the resulting print where the backup roller slides against the printed, bottom face of recording medium during duplex printing.
Still further, the fixing device using a pair of cooling devices is not practical where combined use of the pair of opposed belt assemblies may complicate control of the rotary belts for maintaining synchronized movement of the belts with each other, the absence of which would result in image defects, such as smearing or shifting, of the resulting print.